In fatty liver disease, the death of hepatocytes is an important occurrence because cell death can trigger downstream events such as hepatic inflammation and fibrosis. Certain fatty acids, particularly long-chain saturated species, kill hepatocytes directly and contribute to steatohepatitis in experimental animals. The mechanism by which saturated fatty acids kill cells is not uniform among all cell types and for hepatocytes the process is incompletely understood. The objective of this research plan is to clarify the cellular events involved in saturated fatty acid-induced hepatocyte [unreadable]lipotoxicity.[unreadable] Preliminary data indicate that saturated fatty acids block the mammalian target of rapamycin complex I (mTORCI) in hepatocytes and activate protein phosphatase 2A (PP2A). We propose that these two events are linked, because mTORCI typically maintains PP2A in an inactive state. Furthermore, we propose that events occurring downstream of PP2A activation, namely, suppression of the cyclin-dependent kinase inhibitor p27 and activation of cyclin-dependent kinases, are responsible for saturated fatty acid-induced cell death. We will address these hypotheses in four Specific Aims. In Aim 1 we will investigate how fatty acid-mediated inactivation of mTORC1 causes activation of PP2A, concentrating on the role mTOR in controlling the activity of &#945;4, a negative regulator of PP2A. In Aim 2 we will test the hypothesis that fatty acid-mediated activation of PP2A prompts proteasomal degradation of p27 in hepatocytes, leading to cell death. In Aim 3 we will assess how p27 deficiency leads to cell death, focusing on cyclin-dependent kinases and their effects on mitochondrial integrity and autophagy. Finally, in Aim 4 we will assess whether the pathophysiologic pathway to lipotoxicity in cell culture is also operative in the liver in vivo, by modulating mTORC1, PP2A, p27 and cyclin-dependent kinases in a mouse model of SFA-mediated liver injury and determining their impact on liver-related outcome. The hope is that these studies will shed sufficient light on the pathogenesis of hepatocyte death in the setting of hepatic steatosis to suggest therapeutic options for patients with fatty liver disease.